wq_has_sleeper — check if there are any waiting processes
bool wq_has_sleeper ( | struct socket_wq * wq); |
Returns true if socket_wq has waiting processes
The purpose of the wq_has_sleeper and sock_poll_wait is to wrap the memory barrier call. They were added due to the race found within the tcp code.
CPU1 CPU2
sys_select receive packet
... ...
__add_wait_queue update tp->rcv_nxt
... ...
tp->rcv_nxt check sock_def_readable
... {
schedule rcu_read_lock;
wq = rcu_dereference(sk->sk_wq);
if (wq && waitqueue_active(wq->wait))
wake_up_interruptible(wq->wait)
...
}
The race for tcp fires when the __add_wait_queue changes done by CPU1 stay in its cache, and so does the tp->rcv_nxt update on CPU2 side. The CPU1 could then endup calling schedule and sleep forever if there are no more data on the socket.